Plastic changes in the adrenergic innervation of the rat septal area in response to denervation

Abstract

The adrenergic innervation of the rat septal area was studied using the Falck-Hillarp histofluorescence method in normal animals and in ones in which the septum had been denervated by a unilateral hippocampal lesion. In the normal septum adrenergic terminals are found in all of the principal septal nuclei with the heaviest innervation apparent in the lateral septal nucleus, the nucleus accumbens and the interstitial nucleus of the stria terminalis. A smaller but significant innervation is also present in the medial septal nucleus, nucleus of the diagonal band, posterior septal nucleus and the hippocampal rudiment. Following a total, unilateral transection of the hippocampal fimbria, there is a marked increase in the adrenergic innervation of those septal nuclei that receive a significant projection from the hippocampus. This first appears between 8 and 15 days after operation, becomes marked by 30–60 days, and persists for at least 100 days after the septal denervation. The increase in the number of adrenergic terminals is initially found in the caudal part of the lateral septal nucleus and it subsequently is evident through most of the lateral septal nucleus, the medial septal nucleus and the posterior septal nucleus. No changes were noted in the other nuclei of the septal area. Concomitant with the histochemically demonstrable increase in adrenergic innervation, there is a biochemically detectable increase in the norepinephrine content of the septum. The source of the adrenergic innervation in both the normal and denervated septum appears to be axons traversing the medial forebrain bundle since section of this tract produces a substantial reduction in the adrenergic innervation of the septal nuclei. It is concluded that the increase in norepinephrine-containing terminals produced by removal of the non-adrenergic hippocampal afferents is due to the formation of new terminals which have sprouted from intact norepinephrine-containing axons normally innervating the septal nuclei.